In the exhaust systems of internal combustion engines, particularly those used in automobiles, corrosion from the internal and external surfaces has conventionally been suppressed by the use of ordinary steel plated with aluminum or zinc. However, since exhaust systems are now equipped with catalytic converters or the like for purifying the exhaust gas to reduce environmental pollution, the corrosion resistance provided by such plated steels has become inadequate. As taught, for example, by Japanese Patent Public Disclosures No. Sho 63-143240 and 63-143241, therefore, steels containing 5-10% of Cr were developed for increasing the corrosion resistance of the steel base material. In response to recent increases in the service life and warranty period of automobiles, however, large amounts of high-grade stainless steels containing around 18% of Cr, with or without addition of Mo, are now being used in exhaust systems. Even such high-grade stainless steels have been found to be insufficient in corrosion resistance and subject to localized corrosion (pitting) and the like. Moreover, since the large amounts of Cr and Mo contained in such high-grade stainless steels degrades their processability, they are extremely difficult to fabricate into complexly shaped components such as those used in exhaust systems, and the highly complicated manufacturing processes required increases the processing cost. They may even be completely unusable for some shapes and, at any rate, are expensive as a raw material.
When a steel with a given Cr content becomes more susceptible to pitting owing to a more severe operating environment, as is typical in exhaust systems, the problem is generally coped with by further increasing the Cr or Mo content so as to strengthen the resistance to corrosion.
In light of these circumstances, the object of the present invention is to provide a steel which exhibits strong resistance to a corrosive environment such as present in the exhaust system of an internal combustion engine, exhibits excellent processability, and is low in cost.